Gauns, Mangesh

National Institute of Oceanography, Dona Paula, Goa 403 004, India, E-mail: gmangesh@darya.nio.org

Microzooplankton distribution and its role in the food web of the Arabian Sea

The ecological role-played by microzooplankton in the central and eastern Arabian Sea was studied during three seasons’ viz., spring intermonsoon, winter, and summer. Other biological parameters measured included phytoplankton abundance, biomass (Chlorophyll a) and production; abundance of bacteria and production and mesozooplankton abundance and biomass. Microzooplankton comprising of protozoan [(loricates and aloricates), heterotrophic dinoflagellates and sarcodines] formed 55-96% compared to micro metazoa. Among protozoa, heterotrophic flagellates were abundant (avg. 47%) followed by loricates (27%), aloricates (21%) and sarcodines (5%). Nauplii and copepodite stages dominated micro metazoa. Seasonal averages of microzooplankton were 700 l^-1, 130 l^-1 and 310 l^-1 respectively during spring intermonsoon, winter and summer season. However, a maximum of 5000 l^-1 was registered during summer at near surface. They were generally higher in the upper 100m-water column in all the three season.

Microzooplankton carbon during three different seasons varied between 8 µg C l^-1 (summer) and 30 µg C l^-1 (spring intermonsoon). This accounted for 23-50% of the total living carbon suggesting that they act as a major source of food to higher trophic levels particularly during spring intermonsoon. Overall, coastal stations had higher microzooplankton biomass (21 µg C l^-1) than the oceanic waters (14 µg C l^-1). Interestingly, higher microzooplankton biomass occurred during spring intermonsoon (avg.25 µg C l^-1) when phytoplankton production (avg. 2 mg C m^-3 d^-1) and biomass (avg. 6 µg C l^-1) was low, but bacterial biomass (avg. 7 µg C l^-1) and production (0.8 mg C m^-3 d^-1) were highest. This signified the presence of high amounts of dissolved organic carbon and predominance of a ‘microbial loop’ during spring intermonsoon. At times microzooplankton biomass even exceeded that of mesozooplankton and is a pointer that often microzooplankton dominate the food web in grazing in the Arabian Sea.